The operational buffer in the source driver for thin film transistor liquid crystal display (TFT LCD) is required to have:
(1) high slew rate such that the driving capability is strong enough to precharge the resistor-capacitor (RC) load of the LCD panel to a predetermined voltage level in a predetermined time period;
(2) low quiescent current to save power as much as possible under the condition of being able to drive the RC load of the LCD panel;
(3) low offset voltage for supplying precise driving voltage on the LCD panel for better display performance; and
(4) excellent stability when the above requirements are all satisfied since the operational buffer is always configured with negative feedback.
However, as shown in FIG. 1, in a conventional class AB operational buffer 100, to reduce the offset resulted from the differential input pair 102, there must be introduced with cascade transistors M5 and M6 to increase the direct current (DC) gain of the first stage circuit 101, which will also require larger Miller capacitor 104 to improve the stability of the operational buffer 100. Also due to the insertion of the cascade transistors M5 and M6, the path for current mirroring from the node 106 to the node outp is lengthened, thereby resulting in mismatched mirroring thereof, and the stability is degraded. Moreover, in the operational buffer 100, even though the nodes 110 and outn could be biased at a same voltage, the quiescent current of the output stage 103 will be proportional to the current of the first stage circuit 101 accordingly and therefore, higher stability and less power consumption can never be achieved at same time, resulting in poor design flexibility. In addition, the parasitic capacitance seen from the currents flowing into the nodes 106 and 108 are so large that the phase thereof are significantly shifted, and the stability is decreased accordingly. Furthermore, the operational buffer 100 needs three bias voltages V1, V2 and V3, and the bias circuit therefore (not shown in the figure) will be thus more complicated.
Therefore, it is desired a better class AB operational buffer to avoid the above disadvantages.